Conventionally, there are known gas engines that are operated under lean-burn conditions that realize highly efficient and high-power operation with low NOX emission. In the operating region of a gas engine, there exist a knocking region and a misfire region based on the relationship between air-fuel ratio (excess air ratio) and brake mean effective pressure (BMEP). Therefore, it is desired to control a gas engine so that the gas engine can operate with high efficiency and high power while preventing knocking and misfire.
For example, Patent Literature 1 discloses a gas engine controlling method, in which the ignition timing is adjusted such that a delay calculation value of an actual knocking occurrence ratio becomes a target occurrence ratio. In the controlling method, it is determined, for each cycle, based on the pressure in the cylinder whether or not allowable knocking has occurred. The actual knocking occurrence ratio means the ratio of the number of cycles in which allowable knocking has occurred to a predetermined number of cycles. In this controlling method, if the delay calculation value of the actual knocking occurrence ratio (e.g., a moving average value) is lower than the target occurrence ratio, the ignition timing is advanced based on a deviation between the delay calculation value of the actual knocking occurrence ratio and the target occurrence ratio. If the delay calculation value of the actual knocking occurrence ratio is higher than the target occurrence ratio, the ignition timing is retarded based on a deviation between the delay calculation value of the actual knocking occurrence ratio and the target occurrence ratio.